The position of a radio equipment is information that needs to be known in the context of numerous applications. By way of illustrative examples, the “push” type services supplying user's mobile terminals with contextual information according to their position, guide services for the person in a place like a museum, or even the routing of data packets through a meshed network or an ad hoc network can be mentioned.
Among the various existing positioning techniques, there are those that are based on a cooperative calculation of distance between radio equipments (“ranging”) simply by measuring the time of flight of the radio signal. Knowing the speed of propagation of the electromagnetic wave, the measurement of the time of flight between two radio equipments makes it possible to work out the straight-line, or direct-line, distance separating the two equipments. It should be stated right away that the term “time of flight between two equipments” should be understood to mean the transmission time of the electromagnetic wave to go from one of the equipments to the other (in just one direction). The cooperative distance calculation techniques most commonly rely on the following estimation method:
a) a first equipment sends to a second equipment a request to measure an exchange time Tex between the two equipments and initiates a count of the time,
b) after receiving the request, the second equipment allows a wait time Tw to elapse, the wait time being known to both equipments, then responds to the first equipment by sending an acknowledgement ACK,
c) on receiving the acknowledgement ACK, the first equipment stops counting the time, in order to measure the exchange time Tex, then calculates the time of flight TOF from the first to the second equipment by subtracting the time delay Tw of the measured exchange time Tex and by dividing the result of this subtraction by two,
d) knowing the time of flight TOF and the speed of propagation of the electromagnetic wave V, the first equipment calculates the straight-line distance D between the two equipments by dividing the exchange time Tex by the speed V, the assumption being that the radio signals (go and return) of the exchange, the duration Tex of which has been measured, have taken direct-line propagation paths.
Thus, the estimation of the time of flight TOF between the first and the second equipment results from a measurement of the duration of an exchange, that is, a round trip, between the two equipments. Such a round-trip measurement is necessary in the case where the two equipments are asynchronous, that is, not synchronized on a common clock, and do not therefore share the same time reference. Should the two equipments be synchronous, it would be sufficient to measure the duration of a transmission from one equipment to the other, in just one direction.
The method of estimating the time of flight TOF, as explained previously, gives good results when the two equipments are remote from each other. Indeed, in this case, the assumption according to which the signals of the exchange, the duration Tex of which is measured, follow the propagation paths, go and return, in direct lines, is an acceptable approximation, which makes it possible to estimate the straight-line distance D between the two equipments with good accuracy. However, when the two equipments are relatively close to each other and in an environment favoring multiple propagation paths, as is the case, for example, in a building, the measurement of the exchange time Tex between the two equipments risks being strongly affected by multiple propagation paths. The result of this is that the assumption according to which the signals of the exchange, the duration Tex of which is measured, take straight-line propagation paths (go and return) becomes a rough approximation, which corrupts the estimation of the straight-line distance D between the two equipments with a significant error. In practice, the exchange time Tex is measured for go and return propagation paths each followed by a main part of the energy of the signal (go signal carrying the request and return signal carrying the acknowledgement). A propagation path followed by the main part of the energy of the signal will hereinafter be called the “strongest path”, in the interests of conciseness. The equipments are synchronized on the strongest path. Now, in the case of propagation of the signal along multiple paths, the strongest path does not necessarily correspond to the straight-line propagation path between the two equipments. In practice, a secondary, weaker, part of the energy of the signal can take a shorter propagation path, in a straight line or very close to a straight line. This secondary part of the signal is received before the strongest path by the receiving equipment. The propagation path taken by the part of the energy of the signal received first will hereinafter be called “first propagation path” or “shortest path”. Finally, in the case where the equipments are close and in an environment likely to give rise to multiple propagation paths, the offset that can exist between the time of arrival of the part of the signal that has taken the strongest path and the time of arrival of the part of the signal that has taken the first path is likely to corrupt the estimation of the straight-line distance D between the two equipments by a significant error.
To correct this error, document WO 2006/072697 proposes a method of measuring the distance between a first and a second radio equipment that proceeds in two phases. In the first phase, the steps a) to d) described previously are carried out so that the first equipment estimates a first time of flight TOF1. In the second phase, the second equipment sends the first a channel sounding frame. This frame is designed for the first equipment to detect, from the different radio signals received corresponding to the sounding frame, the strongest path and the first path. The first equipment can thus estimate a difference in time of flight ΔTOF between these two paths and calculate a second time of flight TOF2, or corrected time of flight, by subtracting the determined difference in time of flight ΔTOF from the first measured time of flight TOF1.
This method of estimating the distance between two radio equipments can be used in an ad hoc network of nodes to enable in particular a new node Z arriving in the network to be located. However, applying this method is very costly in terms of energy consumption for the node Z to be located. In practice, it requires three separate measurements, respectively initiated by three different nodes, here denoted A, B and C. The node Z must consequently send three sounding frames to these three nodes A, B and C respectively. Now, transmitting these sounding frames consumes energy.